Metal plating by chemical reduction with borohydrides



United States Patent Metal Hydrides Incorporated, Beverly, Mass, acorporation of Massachusetts No Drawing. Filed .Ian. 26, 1959, Ser. No.788,715

2 Claims. (Cl. 106-1) This invention relates to chemical deposition ofnickel, cobalt and mixtures thereof upon metals, glass, ceramics andplastics by the reducing action of a borohydride in an'aqueous solutionof a salt of nickel and/0r cobalt.

A process for chemical plating nickel and cobalt upon metallic surfaceshas been proposed which involves the reducing action of hypophosphitesin a solution of a nickel or cobalt salt, a temperature of 90 C. or morebeing used to start the reaction. A process also has been proposedinvolving the reducing actions of hypophosphite and hydrazine, a noblemetal catalyst being used to start the reaction.

Sodium borohydride has been used extensively for effecting reductions ofmany compounds in aqueous solu tion. It is known that the alkali metalborohydrides reduce nickel salts but the reduction product is nickelboride. Thus, the United States patent to H. I. Schlesinger and H. C.Brown No. 2,461,661, column 5, lines 2 to 5, states: One characteristicreduction reaction of the alkali metal borohydrides is the reduction ofnickel sulfate in solution to a black precipitate of nickel boride, NiB.

The present invention is based upon the discovery that under certainconditions the water soluble alkali metal borohydrides, such as sodiumborohydride, potassium borohydride and tetramethylammonium borohydride,are highly effective reducing agents for use in chemical plating ofnickel and cobalt upon both metals and non-metals, such as glass,ceramics and plastics. It has been discovered that if the pH of theaqueous solution of the nickel and/or cobalt salt and the borohydride isadjusted to between about 12.0 and 13.0, the boride formation issuppressed and the reduction product is principally elemental metal. ThepH of the solution is adjusted to between 12.0 and 13.0 by the'presencetherein of an alkaline material, such as ammonium hydroxide. Theprecipitation of the hydroxide of nickel or cobalt is inhibited byconventional means, such as by the presence in the solution of an alkalimetal salt of an organic carboxylic acid having at least two carboxylgroups, such as the alkali metal salt of citric, tartaric, adipic,succinic, malonic, oxalic, glutaric, pimelic or malic acid. An effectiveamount of such inhibitor in terms of normality is at least equal to thenormality of the nickel or cobalt salt.

The present invention, therefore, provides a bath for plating bychemical deposition comprising an aqueous solution of a salt of nickeland/or cobalt having a pH between about 12.0 and 13.0, a Water solubleborohydride and a compound which inhibits the precipitation of nickel orcobalt hydroxide.

In the practice of the invention, any water soluble salt of nickel orcobalt may be used, such as the sulfate, chloride, nitrate, acetate,formate, citrate or tartrate. In place of ammonium hydroxide, alkalimetal hydroxides or quaternary ammonium hydroxides may be used, such assodium hydroxide, potassium hydroxide or tetra- 2,942,990 Patented June28, 1960 methylammonium hydroxide. Any water soluble borohydride may beused, such as sodium borohydride', potassium borohydride ortetramethylammonium borohydride.

While the plating bath of the invention is stable for a limited time atnormal room temperature, it is preferred to prepare a separate aqueoussolution of the borohydride having its pH adjusted to between 12.0 and13.0 by the addition of an alkaline material if necessary anda secondaqueous solution containing the salt of nickel and/or cobalt and havingits pH adjusted to between 12.0 and 13.0 by the presence of ammoniumhydroxide or other alkaline material and also containing a compound,such as sodium citrate, for inhibiting the precipitation of nickeland/or cobalt hydroxide. The plating bath is made up when desired bycombining aliquots of these two solutions in desired proportions.Satisfactory results have been obtained with a plating bath in which theconcentration of the nickel or cobalt salt is between about 0.02- and0.15 normal, the concentration of the borohydride being between about0.05 to 0.60 normal but sufficient to effect substantially completereduction of the nickel or cobalt salt.

In the use of the plating bath of the invention, the plating reactionmay be started by heating the bath to a temperature of about 40 C. Thereaction normally is completed in 30 to 45 minutes, that is, when theborohydride is used up. Adherence of the nickel or cobalt plate isespecially good when the surface plated is an unglazed ceramic. Scrapingof the surface with a knifeedge does not remove the plated metal. Whenplating smooth surfaces, such as other metals, glass or glazed ceramics,the adherence of the plate is improved if the surface is first roughenedwith an abrasive, such as emery paper.

The invention is illustrated by the following examples. In theseexamples the reaction was started by heating the plating bath to atemperature of 40 C. and heating was continued at a temperature in therange between 40 C. and 50 C. for a period of 30 to 45 minutes. In eachexample, the base material was coated with an adherent, continuous plateof the plating metal. The continuity of the -.plate on the non-metallicbase materials of Examples 3-6 was tested by checking their electricalresistance with an ohmmeter. Resistivities of the order of 3 to 10 ohmswere found.

Example Five parts by volume of an aqueous solution 0.148 N in nickelsulfate, 0.264 N in sodium citrate and 7.6 N in ammonium hydroxide werecombined with one part by volume of an aqueous solution 0.93 N inpotassium borohydride. The resulting plating bath was 0.123 N in nickelsulfate, 0.22 N in sodium citrate and 0.155 N in potassium borohydrideand had a pH of 12.3, the molar ratio of BH; ions to nickel ions being1.3. The bath was used for plating nickel on copper.

Example 2 Five parts by volume of an aqueous solution 0.081 N in nickelacetate, 0.146 N in sodium citrate and 6.5 N in ammonium hydroxide werecombined with one part by volume of an aqueous solution 0.93 N inpotassium borohydride. The resulting plating bath was 0.067 N in nickelacetate, 0.121 N in sodium citrate and 0.155 N in potassium borohydrideand had a pH of 12.4, the molar ratio of BH, ions to nickel ions being2.3. The bath was used for plating nickel on mild steel.

Example 3 23 parts by volume of an aqueous solution 0.131 N in nickelchloride, 0.213 N in sodium citrate and 8.5 N

in ammonium hydroxide were combined with one part by volume of anaqueous solution 4.44 N in sodium borohydride and containing about 40percent by Weight of sodium hydroxide. The resulting plating bath was0.125 N in nickel chloride, 0.206 N in sodium citrate and 0.185 N insodium borohydride and had a 'pH of 12.8, the molar ratio of BH, ions tonickel ions being 1.5. The bath was used for plating nickel on unglazedceramic.

Example 4 Five parts by volume of an aqueous solution 0.046 N in cobaltacetate, 0.127 N in sodium tartrate and 6.9 N

in ammonium hydroxide were combined with one part by volume of anaqueous solution 1.32 N in sodium borohydride and 6.8 N in ammoniumhydroxide. The resulting plating bath was 0.038 N in cobalt acetate,0.106 N in sodium tartrate and 0.22 N in sodium borohydride'and had a pHof 12.5, the molar ratio of EH ions to cobalt ions being 5.8. The bathwas used for plating cobalt on glass.

Example 5 12.4, the molar ratio of BH; ions to nickel ions being 2.7.The bath was used for plating nickel on Lucite.

Example 6 Five parts by volume of an aqueous solution 0.105 N 30 incobalt nitrate, 0.213 N in sodium citrate and 8.2 N

in ammonium hydroxide were combined with one part by volume of anaqueous solution 0.93 N in potassium borohydride. The resulting platingbath was 0.087 N in cobalt nitrate, 0.177 N in sodium citrate and 0.155N in potassium borohydride and had a pH of 12.4, the molar ratio of EHions to cobalt ions being 1.8. The bath was used for plating cobalt onunglazed ceramic.

I claim:

l. A bath for plating by chemical deposition consisting essentially ofan aqueous solution of a salt of a metal selected from the groupconsisting of nickel and cobalt in a concentration between about 0.02and 0.15 normal, at borohydride selected from the group consisting ofsodium borohydride, potassium borohydride and tetramethylammoniumborohydride, an alkaline material selected from the group consisting ofammonium hydroxide, an alkali metal hydroxide and tetramethylammoniumhydroxide in an amount sufficient to bring the pH of the solution tobetween about 12.0 and 13.0, and an agent adapted to inhibitprecipitation of the hydroxide of said selected metal, said agent beingselected from alkali metal salts of carboxylic acids having at least twocarboxyl groups and being present in a concentration in. terms ofnormality at least equal to the normality of the salt of saidselectedmetal.

2. A plating bath as described by claim 1 wherein said salt of theselected metal is selected from the group consisting of sulfate,chloride, nitrate, acetate, formate, citrate and tartrate.

References Cited in the file of this patent UNITED STATES PATENTS2,726,170 Warf et a1. Dec. 6, 1955 Notice ef Adverse Decisien inInterference In Interference No. 92,380 involving Patent No. 2,942,990,E. A. Sullivan, Metal plating by chemical reduction with borohydrides,final judgment adverse to the patentee was rendered Aug. 20, 1962, as toclaims 1 and 2.

[Ofiicial Gazette September 18, 1962.]

1. A BATH FOR PLATING BY CHEMICAL DEPOSITION CONSISTING ESSENTIALLY OFAN AQUEOUS SOLUTION OF A SALT OF A METAL SELECTED FROM THE GROUPCONSISTING OF NICKEL AND COBALT IN A CONCENTRATION BETWEEN ABOUT 0.02AND 0.15 NORMAL, A BOROHYDRIDE SELECTED FROM THE GROUP CONSISTING OFSODIUM BOROHYDRIDE, POTASSIUM BOROHYDRIDE AND TETRAMETHYLAMMONIUMBOROHYDRIDE, AN ALKALINE MATERIAL SELECTED FROM THE GROUP CONSISTING OFAMMONIUM HYDROXIDE, AN ALKALI METAL HYDROXIDE AND TETRAMETHYLAMMONIUMHYDROXIDE IN AN AMOUNT SUFFICIENT TO BRING THE PH OF THE SOLUTION TOBETWEEN ABOUT 12.0 AND 13.0, AND AN AGENT ADAPTED TO INHIBITPRECIPITATION OF THE HYDROXIDE OF SAID SELECTED METAL, SAID AGENT BEINGSELECTED FROM ALKALI METAL SALTS OF CARBOXYLIC ACIDS HAVING AT LEAST TWOCARBOXYL GROUPS AND BEING PRESENT IN A CONCENTRATION IN TERMS OFNORMALITY AT LEAST EQUAL TO THE NORMALITY OF THE SALT OF SAID SELECTEDMETAL.